One of the proposed methods for Digital Rights Management (DRM) of high quality audio content involves making only low quality versions available to unlicensed users. Licensed users however are provided with some additional information, typically a cryptographic key, that allows them access to an additional quality layer, so as to obtain the original high quality content. Typically the additional quality layer is provided as a separate and encrypted bitstream, i.e. separate from the low quality bit stream. An example of such a dual layer quality approach can be found in the DRM methods being proposed for DVD-Audio.
The presence of two separate bit streams poses a security risk because the encrypted high quality layer is easily traceable, and therefore accessible for cryptographic attacks. For example, by playing out the high quality layer of DVD-Audio, an attacker can try to exploit the observed relation between the encrypted and decrypted bit streams for retrieving cryptographic keys.